1. Field of the Invention
The present invention relates to polyester and a producing method thereof.
2. Description of the Related Art
In recent years, liquid crystalline polyester having a backbone with an aromatic ring has been used in the electric and electronic fields as a material superior in heat resistance and tensile strength.
Such liquid crystalline polyester is produced, for example, through transesterification of an acylated product with aromatic dicarboxylic acid such as terephthalic acid, the acylated product being obtained by acylating phenolic hydroxyl groups of aromatic hydroxycarboxylic acid such as para-hydroxybenzoic acid and/or of aromatic diol such as 4,4′-dihydroxybiphenyl with acetic anhydride.
In the above-mentioned producing method, however, a high temperature of 250° C. or more is sometimes required for transesterification, and reaction time tends to be long. Long period of reaction time at a high temperature may cause thermal hysteresis of liquid crystalline polyester. In order to improve the method, various attempts have been made. For example, Japanese Unexamined Patent Publication No. 11-246654 discloses a method of shortening reaction time by adding an organometallic compound such as sodium acetate as a catalyst during acylating reaction, and Japanese Unexamined Patent Publication No. 6-1836 proposes a method of adding a low-boiling-point organic compound such as pyridine as a catalyst during acylating reaction.
However, even in the case of adopting the above-mentioned methods, reaction at a high temperature is needed, and therefore thermal hysteresis of liquid crystalline polyester is not suppressed, which causes thermal degradation of the polyester. Such thermal degradation is notable in the case of obtaining aromatic polyester as compared with the case of obtaining aliphatic polyester.